Etching of aluminum



Patented June 12, 1951 UNITED STATES PATENT OFFICE.

ETCHING F ALUMINUM Charles Edmund Meulendyke, Rochester, N. Y.

v No Drawing. Application May 19, 1949, Serial No. 94,260

1 Claim.

This invention relates to the etching of an aluminum or aluminum alloy layer supporting local resists and is of special interest in the arts of metal-decoration and photomechanical print- An aluminum sheet has been extensively used in photo-metallography or the application of the method of photo-lithography to metallic surfaces. However, the advantage of aluminum as an exceptionally light-weight material has been little utilized in other major divisions of the graphic arts. Lack of suitable means of etching appears to have been a deterrent in employment of this useful property. What might be called the conventional method for etching aluminum appears to have been the use of hydrochloric acid with or without addition of a small amount of a metallic chloride to accelerate the etching action. Further, such application appears to have been limited to the etching of coarse designs such as name plates and dials. It is known that suppliers ,of engraving metals have regarded such method as unsuitable for fine etching such as is required for half-tone relief printing or cellular intaglio printing.

Aluminum etches in caustic alkali destructive alike to resinous and colloid resists. Nitric acid has no etching effect whatever on aluminum. Aluminum does etch in hydrochloric acid with evolution of hydrogen gas but the rate is slow compared to the ebulliency of the reaction. There is attack on the resists and disappointingly strong side action in proportion to depth of etching attained. It is an object of the invention to supply a reasonably fast and smooth method for etching aluminum. It is another object to provide a method in which the comparatively tender resists produced from a silver halide emulsion layer are not harmed. It is yet another object to etch aluminum in a simple tray operation without presence of objectionable fumes. It is a primary object to supply a suitable etching method for making aluminum available in the preparation of typographic intaglio printing plates.

Reference is made to U. S. Patent 2,233,546. In such invention it is shown that cupric chloride is a specific etchant for nickel. I have found that cupric chloride is a specific etchant for aluminum. The action is reasonably fast and results very smooth particularly when accompanied by a casual brushing action. I have further found that cupric chloride in pure alcohol solution is not a suitable etchant for aluminum as the copper tends to plate out with undesirable adhesion and with little accompanying displacement of aluminum; that salts of copper other than the cupric halides have little or no etching action on aluminum; that other metallic chlorides excepting ferric chloride have little or no etching'action on aluminum; and that ferric chloride etches rather too slowly to be useful.

The mechanism of the primary reaction between aluminum and cupric chloride appears to involve separation of copper in colloidal form and dissolution of the aluminum as aluminum chloride. However, there is considerable evolution of hydrogen gas and active display of galvanism from copper-aluminum coupling. It is especially advantageous that the colloidal copper formed shows but limited tenacity of sticking to the aluminum in marked contrast to its behavior toward zinc.

A water-solution of cupric chloride by and of itself tends to react too violently. It is advantageous and rather necessary to add tempering controls. A viscosity-increasing ingredient such as glycerine can exercise a general control over speed of reaction, heating, protection of resists, colloidal condition of the precipitated copper and sticking of the copper to the aluminum. I find in orthophosphoric acid a particularly valuable tempering control. It is of a highly syrupy nature. It is not an etchant of aluminum but appears to act as a highly satisfactory buffer to the action of the cupric chloride.

'As a specific example of an etching solution for the etching of aluminum I suggest equal parts by volume of a 40% water-solution of cupric chloride, glycerine, and the orthophosphoric acid of commerce. Upon mixing, the cupric chloride may for a precipitate. Water is added in very small amounts with stirring until solution becomes clear. The exemplary solution which has considerable viscosity is applied with a brush and in successive small portions. It is advisable to place the aluminum sheet upon a chilled surface. I find that chemically the method has general application to the general range of commercial wrought alloy aluminum sheets. Pure aluminum is soft and easily bent but small additions of alloying elements such as managense, chromium, and nickel give the requisite physical properties of hardness and rigidity.

Aluminum per se tends to passivate its surfaces with a layer of oxide from reaction with atmospheric oxygen. The etched plate of the invention with resists removed is in a favorable condition for having its surfaces hardened with a protecting layer of aluminum oxide by known anodic treatment. This is advantageous in preparing plates for rotogravure to withstand wear and tear from the doctor-blade.

It may be advantageous to employ a solution of nitric acide to remove last traces of adhering copper remaining from the etching operation. The aluminum itself is entirely inert to nitric acid.

While I do not limit myself as to the type of plate to be prepared by the method of the invention, such method appears to be more favorably adapted to formation of shallowly etched intaglios than of deeply etched rilievos. The method appears to be especially advantageous in the preparation of intaglio typographic plates as dissociated from and obviating need for an initial setting-up of type. The particular character of typographic image which I envisage for such purpose is one bearing ink-filling cellular structure analogous to photogravure.

20 It will thus be seen that I have described methods with all necessary details embodying the principles and attaining the objects and advantages of the invention. Since many matters of 4 treatment, manipulation, selection of materials, succession of steps and other details may be variously modified without departing from the principles involved, I do not intend any limitation to such details excepting so far as set forth in the appended claim.

What is claimed is:

In the arts of metal-decoration and photomecham'cal printing, the method of etching aluminum by applying a solution containing cupric chloride, glycerine and orthophosphoric acid to an aluminum sheet supporting local resists.

CHARLES EDMUND MEULENDYKE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,740,731 Gravell Dec. 24, 1929 2,127,885 Pettit Aug. 23, 1938 2,233,546 Meulendyke Mar. 4, 1941 2,318,322 Meulendyke May 4, 1943 

